Is 10 mg in 2.5 mL of 0.9% sodium chloride a sensible presentation for cagrilintide?

The case in front of me: 10 mg · 2.5 mL · 0.9% sodium chloride · cagrilintide.

I want to decide this in advance so that I am not deciding it under pressure later.

Assume I will follow the plan I write down, so I would like it to be a good one.

How would you structure this, and what thresholds would you set in advance?

Answering this needs the syringe you actually own, because the barrel graduations decide what "readable" means.

Vial headspace is the hard constraint. A nominal 2 mL vial typically holds a little over 2 mL to the shoulder; adding 3 mL is not an option and attempting it wastes the lot.

For a dose that will change during titration, choose the volume for the largest intended dose rather than the first, so the whole schedule fits on one barrel without a mid-vial recalculation.

A concentration calculated to three decimal places from a diluent volume measured to one is false precision.

Choose the volume that puts your largest intended dose between 10 and 30 units. Everything else follows.

edited 6 May 2025 by lyoph_cake — fixed an arithmetic slip in the third paragraph

Answer first: diluent volume sets concentration and therefore resolution on the syringe barrel, and resolution is free at reconstitution and impossible to recover afterwards.

Dead-space loss scales with the number of draws, not with the concentration, so a lower concentration spread over more draws loses proportionally less of the total peptide.

Write the concentration and the resulting units-per-dose on the vial label at reconstitution. The arithmetic that is obvious now will not be obvious at six in the morning three weeks from now.

Do not change the diluent volume between vials of a titration without recalculating; it is the commonest source of a ten-fold error.

Concentration equals content over volume, and content is not label claim.

The honest answer is that a wide range of volumes works and that the extremes at either end cause avoidable problems.

Round to a diluent volume you can measure accurately. Measuring 1.00 mL on a 1 mL syringe is reliable; measuring 1.37 mL on anything is not, and the error propagates into every dose.

Nominal vial volumes in the standard 2R and 3R glass sizes have published brimful capacities well above the nominal fill, but the usable volume is bounded by the stopper displacement.

Check the vial can physically hold the volume before you draw it up.

Stated carefully, the relevant arithmetic is concentration equals vial content divided by diluent volume, and content is not the same as label claim.

Worked example. A 10 mg vial reconstituted with 2 mL gives 5 mg/mL. A 0.5 mg dose is 0.5 ÷ 5 = 0.1 mL, which on a U-100 syringe is 10 units. Reconstitute the same vial with 1 mL and the concentration doubles to 10 mg/mL, the same dose becomes 0.05 mL, and you are now reading 5 units instead of 10 — the same dose at half the resolution.

U-100 means 100 units per millilitre by definition, so 1 unit is 0.01 mL and volume in millilitres times one hundred gives units. Every conversion here reduces to that.

The caveat is that this arithmetic assumes the vial contains what the label says, and without a content assay it is precise about an unknown quantity.

Measure a volume you can actually measure. Round numbers, real syringes.